The present invention relates generally to a motor control circuit and more particularly to a motor control circuit employing improved means to limit motor current.
Many motor control circuits, well known in the art, act to govern motor operation and include means to limit the motor current. It is also known that certain motors, particularly d.c. motors, can be controlled by varying the effective applied power. Perhaps the most common method today of varying effective power supplied to a motor, at least with respect to larger motors, employs what is known as phase control of a controlled rectifier bridge (commonly silicon controlled rectifiers) which is disposed between an alternating current source and motor. In a phase controlled bridge, the power is varied by controlling the point in time relative to the a.c. voltage at which each rectifier of the bridge is rendered conductive. When the control system further includes current limiting means, it is known that one way of achieving the limiting function is by using the current limiting means to control, during those periods when the motor current is at its limit, the point in time at which each controlled rectifier is rendered conductive.
One such system as has been outlined above is described in U.S. Pat. No. 3,526,819, "Current Limit For Motor Control Systems" by Carlton E. Graf, issued Sept. 1, 1970 and assigned to the assignee of the present invention. In this patent a signal proportional to the motor current is compared to a fixed value or reference such that when the motor current exceeds that value the firing of the next rectifier in the bridge is delayed until such time as the motor current falls below the value. This system provides a very fast response current limit which is quite satisfactory in most instances. When the motor is running at its current limit, the firing of the controlled rectifier bridge is solely under the control of the current limit circuit. However, in certain instances, particularly when there is a sudden loading of the motor at high regenerative voltages, instability can occur resulting in a motor current which varies rather severely and which tends to render motor operation somewhat erratic.